Chapter 1. Prenatal Testing

Synopsis

Human Development Video Activity

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You must read each slide, and complete any questions on the slide, in sequence.

amniocentesis

An invasive prenatal test in which a needle is inserted through the mother's abdominal wall and into the amniotic sac surrounding the fetus to draw a sample of amniotic fluid for testing.

chorionic villus sampling

(CVS) An invasive prenatal test in which either a needle is inserted through the mother’s abdomen or a catheter is inserted through the vagina and cervix to draw a sample of placental tissue that is tested for more than 200 chromosomal diseases.

Doppler device

A small, non-invasive, diagnostic instrument that uses high-frequency sound waves, which bounce off the moving blood cells, to measure blood flow and blood pressure. A Doppler device can provide information about the baby’s circulation and often provides an audible simulation of the baby’s heartbeat.

Down syndrome

The most common chromosomal disorder characterized by various mental and physical symptoms, such as delayed mental and physical development, decreased muscle tone, heart abnormalities, language difficulties, and learning problems. Individuals with Down Syndrome have 3, instead of 2, copies of chromosome 21.

fetoscopy

An invasive prenatal procedure in which a thin, flexible device is inserted through the mother’s abdomen to view the fetus and the placenta and/or to perform surgical procedures on the fetus during pregnancy.

genetic counseling

The testing and counseling of prospective parents about their genetic potential to pass on disorders and conditions to their offspring.

phenylketonuria

(PKU) A rare, inherited disorder that prevents the body from breaking down a specific amino acid. If left untreated, this disorder can lead to serious health problems, such as brain damage, seizures, behavioral or social problems, and progressive mental retardation.

sonogram

An image generated from a noninvasive diagnostic imaging technique that uses the high-frequency sound waves of ultrasound. While this technique may be used for any part of the body, it is often used on a pregnant woman's abdomen and pelvic cavity to generate an image of a baby and the placenta. While the terms ultrasound and sonogram have different meanings (ultrasound refers to the medical technique and sonogram refers to the image generated from this technique), the terms are often used interchangeably.

sickle-cell anemia

A hereditary blood disorder that affects red blood cells causing pain and infections. It is found largely in the African American population.

spina bifida

A congenital birth defect in which the bones of the spine do not properly form around the spinal cord during prenatal development. Depending on the severity of the spina bifida, there may be lifelong physical limitations and neurological symptoms caused by the poorly developed spinal column.

Tay-Sachs disease

A rare, inherited, degenerative, untreatable disorder of the nervous system most commonly found in those of Eastern European Jewish descent. In infants and children, the disease is fatal and progresses rapidly as it destroys nerve cells in the brain and spinal cord. Very rarely, this disease can appear in adults and while not necessarily fatal in adults, it leads to massive degeneration of the central nervous system.

ultrasound scan

A noninvasive diagnostic imaging technique that uses the high-frequency sound waves of ultrasound to generate an image. While this technique may be used for any part of the body, it is often used on a pregnant woman's abdomen and pelvic cavity to generate an image of a baby and the placenta. While the terms ultrasound and sonogram have different meanings (ultrasound refers to the medical technique and sonogram refers to the image generated from this technique), the terms are often used interchangeably.

Prenatal Testing

Author

Pregnant woman receiving a ultrasound, or sonogram. A doctor or technician is sliding the ultrasound instrument, called a transducer, over the woman’s bare pregnant stomach.

Catherine Robertson, Grossmont College
S. Stavros Valenti, Hofstra University

Synopsis

This activity focuses on several prenatal tests, some of which determine the general health of the mother and child and others search for genetic abnormalities in the developing fetus. In this activity, you will learn about some specific noninvasive and invasive tests that are available to parents before the birth of their child.

REFERENCES

Mennuti, M. (1997). Information about Chorionic Villi Sampling. University of Pennsylvania Health System. Available at: http://www.uphs.upenn.edu/obgyn/divisions/ChorionicVilliSampling.doc

Public Broadcasting System (PBS), (2001). Windows on the womb. From Nova: Life’s Greatest Miracle. Available at: www.pbs.org/wgbh/nova/miracle/wind_nf.html

Wingerson, L. (1998). Unnatural selection: The promise and power of human gene research. New York: Bantam Doubleday Dell.

Types of Prenatal Testing

Two parents looking at ultrasound photo image of their unborn child

Prenatal tests fall into two general categories: noninvasive tests and invasive tests. An invasive test is a procedure that requires either a medical instrument or needle be actually inserted into the mother’s uterus. In contrast, a noninvasive test may be performed without invading the mother’s uterus.

We can also categorize prenatal tests according to their general goal.

1. Routine tests that assess health risks to either the mother or the child

Examples of these routine tests include blood tests, blood pressure readings, and urine analyses, among others. Blood tests can reveal maternal diseases or drugs, medications, or other toxins that the mother has ingested that are potentially harmful to the fetus. Blood pressure readings can indicate hypertension which is often linked to toxemia, a build-up of toxins in the blood. Urine analyses can measure protein levels. An excess of protein can also be a flag for toxemia and other health concerns for both the mother and fetus.

2. Prenatal tests designed to assess the presence of serious genetic problems in the fetus

Candidates for this category of prenatal tests include some the following: couples who have had problems with infertility, spontaneous abortion, or stillbirth; pregnant women over 34 years of age because of the greater risk of their having children with Down syndrome; couples with family histories of genetic conditions such as sickle-cell anemia, Tay-Sachs disease, or phenylketonuria. Sometimes couples in specific ethnic groups volunteer for these tests during genetic counseling, which are sessions that enable individuals to learn about their genetic heritage as well as any conditions that might affect future children.

Noninvasive Tests: The Alpha-fetoprotein Assay

A pregnant woman getting blood drawn in a doctor’s office

The alpha-fetoprotein (AFP) assay is a blood test performed on the mother to determine the level of alpha-fetoprotein, a substance produced by the baby’s liver and transported to the mother’s bloodstream.

Purpose: This test is to determine if the level of alpha-fetoprotein is within the normal range. High levels of AFP may indicate that the baby has a neural tube defect, which would indicate a deficiency in the amount of brain material present. A lower level of AFP may indicate Down syndrome.

Timing: The test is performed between the 15th and 18th weeks of pregnancy.

Level of Risk: AFP assays pose no risk for either the mother or the fetus.

Recommendations: If levels fall outside of the normal range, further testing may be required. These further tests may include a sonogram and an amniocentesis.

Noninvasive Tests: The Doppler Device

A pregnant woman with two Doppler devices gently strapped around her at the top and bottom of her pregnant belly

The Doppler device is a small, portable machine that uses ultrasound waves to magnify and locate the fetal heartbeat. This test may normally be performed during a regular doctor’s visit.

Purpose: A Doppler test is used to assess that the baby is alive and progressing at a normal rate. This test will allow the physician to hear the fetal heartbeat and to determine whether the heart rate is within the normal range (between 120 and 170 beats per minute).

Timing: This test may be performed as early as the 6th week of pregnancy or as late as just before delivery.

Level of Risk: Doppler tests pose no risk to either the mother or the fetus.

Recommendations: If the heart rate is not within the normal range, the doctor may recommend a sonogram. If a sonogram indicates further concern, other tests, including a fetoscopy, may be performed.

The sound that you are hearing is a fetal heartbeat as heard through a Doppler device.

Noninvasive Tests: The Sonogram

An ultrasound scan, known as a sonogram, uses high-frequency sound waves that bounce, or echo, off the fetus to generate a “moving” picture of the baby. Three-dimensional ultrasound machines give an even better look at the developing fetus.

Purpose: A sonogram allows a physician to look for the presence of neural tube defects or anatomical anomalies. A sonogram also helps the doctor to estimate delivery date and to see if there are multiple fetuses, to measure the fetus’s rate of growth, and to determine the sex of the child. At later points in the pregnancy, a physician uses a sonogram to check the placement of the placenta, the amount of amniotic fluid, and whether the baby is at an appropriate level of growth and development.

Timing: Sonograms may be performed anywhere from the 5th week of pregnancy to full term. Most physicians schedule the screening between the 16th and 18th weeks of pregnancy.

Level of Risk: Sonograms pose no risk to either the mother or the fetus.

Recommendations: If the sonogram indicates a problem, the physician would call for further testing, probably starting with an amniocentesis.

Sonogram at 2 1/2 months.

Sonogram at 3 months.

Noninvasive Tests: The Sonogram (continued)

Sonogram at 4 1/2 months.

Sonogram at 6 months.

Invasive Tests: Amniocentesis

Diagram of an amniocentesis procedure. An ultrasound probe is used to locate the fetus and amniotic fluid within the womb and a needle is inserted to withdraw a sample of amniotic fluid for testing.

Amniocentesis is an invasive technique in which a physician uses a sonogram as a visual guide to insert a long hollow needle through the mother’s abdominal wall to withdraw a small amount of amniotic fluid from the sac surrounding the fetus. The fluid collected during amniocentesis yields a combination of cells and other substances found in the amniotic sac that can be examined for genetic testing and chromosomal abnormalities.

Purpose: Amniocentesis can determine the presence of chromosomal abnormalities, which may indicate Down syndrome and/or other genetic diseases. Amniocentesis is routinely performed on women considered at high risk for such chromosomal abnormalities or disease either because of their age or family histories.

Timing: Amniocentesis is usually performed between the 16th and 18th weeks of pregnancy.

Level of Risk: Spontaneous abortion occurs in approximately 1 out of every 200 to 400 amniocentesis tests.

Recommendation: If an abnormality is indicated, the physician and parents will have a discussion to explore the types of medical procedures, if any, available for treating the abnormality during pregnancy or immediately following birth. If the fetal abnormality is deemed fatal and not treatable, the discussion might turn to terminating the pregnancy among parents and physicians who consider this an option.

Invasive Tests: Chorionic Villus Sampling

Diagram of chorionic villus sampling. An ultrasound probe is used to locate the fetus within the womb and a catheter is inserted to guide an instrument into the uterus to withdraw a sample of placenta for testing.

Chorionic villus sampling is an invasive test in which a physician uses a sonogram as a visual guide to insert an instrument either through the mother’s abdominal wall or vaginally through the mother’s cervix to remove a small sample of placental tissue from the chorion membrane surrounding the amniotic sac. The advantage of this test is that it can be performed earlier than an amniocentesis. The disadvantages are that the results of chorionic villus sampling are not as accurate as those from an amniocentesis and the risk to the mother and fetus is greater.

Purpose: Chorionic villus sampling screens for genetic disorders, such as Down syndrome and other chromosomal abnormalities, for mothers considered at risk for fetal genetic disorders because of age or the parents’ family history.

Timing: Chorionic villus sampling is usually performed between 10 1/2 and 13 weeks.

Level of Risk: Spontaneous abortion occurs in approximately 1 to 2 out of every 100 tests.

Invasive Tests: Fetoscopy

Diagram of a fetoscopy procedure. An ultrasound probe is used to locate the fetus within the womb and a fetoscope is inserted through the abdomen to take images of the fetus.

Fetoscopy is an invasive test that is usually scheduled as a follow-up procedure after a sonogram or an AFP assay indicates a malformation, such as spina bifida. To perform the procedure, a physician inserts a fetoscope, which is a long, very narrow tube with a viewing instrument, through the mother’s abdomen and uterine wall. Fetoscopies let physicians observe the fetus and placenta directly and, with additional tools, take samples of placental blood, umbilical cord, or fetal liver tissue for analysis.

Purpose: Fetoscopy can be used to verify results of earlier tests that have indicated the presence of a genetic abnormality.

Timing: Fetoscopies are performed around week 18 after a sonogram or alpha-fetoprotein assay indicates a malformation, such as spina bifida.

Level of Risk: Spontaneous abortion occurs in approximately 1 out of every 50 tests.

Risks and Benefits of Prenatal Testing

Prenatal tests do not make ethical decisions. They merely offer couples more information about their baby’s health. In the end, the couple, or in some cases the single mother, must decide how to use the information revealed by prenatal tests while taking into consideration input from the physician and other trusted family members, friends, and counselors.

We should not lose sight of the enormous benefits of prenatal testing. They provide an opportunity for early treatment, which is sometimes as simple as a special diet for the mother. Prenatal tests make it possible for parents having a child with serious abnormalities or a fatal condition to decide whether to continue the pregnancy or to end it. Prenatal tests give the gift of time to parents who, despite early warnings of serious prenatal problems, have decided to continue a pregnancy. In the few remaining months before birth, these parents can attempt to adjust to and prepare for the challenges that they will face in raising their child.

Assessment: Check Your Understanding

Question 1.1

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Correct! Chorionic villus sampling is an invasive test, and the risk of spontaneous abortion is about 1 percent. This test would not be part of routine screening although it might be ordered if other tests have indicated a problem.

Incorrect. Chorionic villus sampling is an invasive test, and the risk of spontaneous abortion is about 1 percent. This test would not be part of routine screening although it might be ordered if other tests have indicated a problem.

Assessment: Check Your Understanding

Question 1.2

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Correct! An alpha-fetoprotein (AFP) assay is a blood test that is performed during the 16th and 18th weeks of pregnancy.

Incorrect. An alpha-fetoprotein (AFP) assay is a blood test that is performed during the 16th and 18th weeks of pregnancy.

Assessment: Check Your Understanding

Question 1.3

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Well done! High levels of AFP do indicate neural tube defects.

Sorry. This is a true statement. High levels of AFP do indicate neural tube defects.

Assessment: Check Your Understanding

Question 1.4

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An ultrasound scan, or sonogram, poses no risk to mother and fetus. This test can detect neural tube defects and anatomical anomalies, multiple fetuses, rate of fetal growth, and the developing fetus’s sex. At a later point in pregnancy, this test can determine the placement of the placenta and the amount of amniotic fluid. A 4D ultrasound provides a three-dimensional image that moves! It is a 3D image taken over time so that you can capture the movements of the baby in utero. In many cases, a 4D is not generally medically necessary, but it does make a nice momento for the family.

Assessment: Check Your Understanding

Question 1.5

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Discuss the higher rate of Down syndrome in women who are age 34 or older. You should also discuss the various tests that can indicate the presence of Down syndrome, such as AFP assays, amniocentesis, and chorionic villus sampling. You should point out that these tests not only show the presence of Down syndrome but can also rule it out, which would allow your friend to abandon any worries that she might have on this topic.

Congratulations! You have completed this activity.Total Score: x out of x points (x%) You have received a provisional score for your essay answers, which have been submitted to your instructor.